Solid-state electronic heterojunctions at the solid-electrolyte and electrode interfaces play a critical role in determining ionic transport, and therefore the performance of solid-state batteries and fuel cells. The formation of a “space-charge layer” at the solid electrolyte and electrode interface is often cited as a barrier for ion transport, but a comprehensive first-principles modeling approach is still missing. For solid-state-batteries (SSB), predicting the space-charge-layer formation at the solid-electrolyte-and-electrode interface is even more challenging, as ion insertion and/or reaction with the electrode alters the material and thus band alignments at the interfaces.

In this talk, we established a theoretical framework to predict the interface potential profiles from thermodynamic driving forces [1]. We first assumed the electrochemical potential for Li⁺ ions reached a constant at the open circuit equilibrium condition, then derived the relationship among the electrostatic potential, the lithium chemical potential, Fermi level, ionization potential, and the work function. This relationship yielded quantitative profiles of the electrostatic potential and electronic energy level alignments across the entire battery. This model complemented direct microscopic and macroscopic simulations by rigorously and simultaneously determining the potential drop, electrostatic dipole, and space-charge layer at the interface.

The application of this model to the Li/LiPON/LixCoO2 system led to the important discovery that the space-charge layer varies with the state of charge (SOC, i.e. Li concentration in Li_xCoO₂). More specifically we predicted that Li⁺ transferred from LIPON to Li_0.5CoO₂, but transferred from Li_1.0CoO₂ to LiPON, this result unified the seemingly contradictory experimental observations. We also predicted that electron transfer caused the interface dipole that can accelerate or impede Li⁺ flow during the discharge process at the in Li_xCoO₂/LiPON interface, depending on the SOC. This will have a profound impact on the overpotentials and the overall cycling efficiency. An important design rule was suggested that increasing solid electrolytes’ valence band or engineering the interlayer at the interface can overcome the additional barrier caused by the interface dipole.

This modeling framework is general for other solid-state energy storage devices with ionic and mixed conductor interfaces.

[1] Michael W. Swift and Yue Qi, Phys. Rev. Lett. 122, 167701 (2019)